The Effects of Dams on Longitudinal Variation in River

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Journal of Freshwater Ecology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tjfe20 The effects of dams on longitudinal variation in river food webs Ivor Grownsa, Bruce Chessmanb, Simon Mitrovicbc & Doug Westhorpea a New South Wales Office of Water, Armidale, New South Wales 2351, Australia b New South Wales Office of Environment and Heritage, Parramatta, Sydney, New South Wales 2124, Australia c Centre for Environmental Sustainability, School of the Environment, University of Technology, Ultimo, Sydney, New South Wales 2007, Australia Click for updates Published online: 09 Sep 2013.

To cite this article: Ivor Growns, Bruce Chessman, Simon Mitrovic & Doug Westhorpe (2014) The effects of dams on longitudinal variation in river food webs, Journal of Freshwater Ecology, 29:1, 69-83, DOI: 10.1080/02705060.2013.832423 To link to this article: http://dx.doi.org/10.1080/02705060.2013.832423

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The effects of dams on longitudinal variation in river food webs Ivor Grownsa*, Bruce Chessmanb, Simon Mitrovicb,c and Doug Westhorpea

aNew South Wales Ofﬁce of Water, Armidale, New South Wales 2351, Australia; bNew South Wales Ofﬁce of Environment and Heritage, Parramatta, Sydney, New South Wales 2124, Australia; cCentre for Environmental Sustainability, School of the Environment, University of Technology, Ultimo, Sydney, New South Wales 2007, Australia (Received 1 April 2013; accepted 4 July 2013)

We examined the effects of two dams on longitudinal variation of riverine food webs using stable isotope and gut contents analyses along four rivers in the Hunter Valley in eastern Australia. Longitudinal 15N enrichment was observed in most invertebrate taxa and food sources but significant longitudinal variation was rare for 13C, and composition of gut contents of invertebrate taxa did not vary significantly with longitudinal position. Most invertebrates and food sources were more 15N-enriched at sites immediately downstream of the dams than expected from their upstream longitudinal position, a result not mirrored by gut contents and 13C. Enrichment of 15N downstream may be attributed to altered water quality as a result of impoundment but further research is necessary to elucidate whether physico-chemical riverine processes or trophic mechanisms are responsible. Our observations regarding the influence of dams on isotope ratios are contrary to the few existing studies, suggesting the small volumes relative to annual inflows of dams in the present study limit downstream impacts by maintaining aspects of flow variability. Keywords: river regulation; tail waters; dams; water resource management; aquatic macroinvertebrates; stable isotopes

Introduction Understanding longitudinal variation in the contributions of different energy sources to river food webs is important for both ecological theory and river management (Gawne et al. 2007). The River Continuum Concept of Vannote et al. (1980) suggests that the dominant food resources for primary consumers vary in a predictable manner from headwater streams to large rivers. Many authors suggest that allochthonous carbon, derived from terrestrial inputs, is the major source of energy in forested headwater streams Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 (e.g. Gessner et al. 1999; Reid et al. 2008) and that the contribution of autochthonous carbon increases with river size (Finlay 2001; Hadwen et al. 2010a). However, natural longitudinal patterns of energy ﬂow can be disrupted by the presence of dams in a river system. The Serial Discontinuity Concept (Stanford & Ward 2001) stresses the recovery of ecosystem processes downstream of major longitudinal disruptions with the natural addition of tributary inputs. Nevertheless, downstream recovery can also take place without major tributary contributions (e.g. Growns et al. 2009). Stable isotope analysis (SIA) has commonly been used to reconstruct food webs and energy ﬂow from microbes, plants and detritus to primary and secondary consumers (Peterson & Fry 1987; Hershey et al. 2006). In many ecosystems, individual food sources

*Corresponding author. Email: [email protected]

Ó 2013 Taylor & Francis 70 I. Growns et al.

have different ratios of 13C:12C and 15N:14N; therefore, food assimilation by animals can be inferred from the isotopic signatures of their tissues (Fry 1991). However, when sources have similar or overlapping isotope ratios it can be difficult to estimate their relative contributions. Gut contents analysis (GCA) is also frequently used to investigate diets by revealing the food ingested during a short period prior to sampling (e.g. Whitledge & Rabeni 1997; Mantel et al. 2004; Li & Dudgeon 2008), whereas SIA can indicate which prey items are assimilated in the medium to long term (Perga & Gerdeaux 2005). Therefore, the two techniques are complementary means of assessing trophic linkages (Post 2002). The effects of dams on the structure and function of river food webs are poorly known (Power & Dietrich 2002) and although some studies have investigated the effects, they show contrasting results. Angradi (1993) observed enrichment of 13C and depletion of 15N in epilithon, enrichment of 13C and 15N in seston and a primary consumer, and enrichment of 15N in macrophytes compared to a nearby unregulated tributary. It was suggested that the changes in d13C were due to a decreasing contribution of phytoplankton with downstream distance, but they could offer no explanation for the enrichment of 15Nin macrophytes. In contrast, Angradi (1994) observed depletion of 13C and 15N in seston in a downstream direction and no longitudinal variation in the isotopic composition of amphipods or fish for 25 km. Shannon et al. (2001) observed enrichment of 13C in benthic algae, macroinvertebrates and fish up to 350 km downstream of Glen Canyon Dam but no longitudinal trends in d15N for the same groups. Doi et al. (2008) used isotope signatures of phytoplankton to suggest that they contributed to the downstream food webs for up to 10 km. Overall, while these studies evaluated differences in energy flow between regulated and unregulated rivers, they did not generally account for potential longitudinal patterns in stable isotope composition. Therefore, in order to evaluate natural variability aside from the effect of dams at various downstream locations, it is important to account for longitudinal variation in food-web dynamics simultaneously in regulated and natural, unregulated rivers. In this study, we predicted that the presence of dams would affect invertebrate diets and the isotopic composition of invertebrates, and potential food sources in two regulated rivers. To assess this we used both SIA and GCA to determine the effects of dams on the longitudinal variation in d13C and d15N isotopic composition of food sources, primary consumers (and their gut contents) and invertebrate predators in regulated and unregulated rivers. We compared sites immediately downstream of dams with two unregulated rivers and unregulated sites upstream of the dams to determine whether dams influenced longitudinal variation in food-web dynamics. Downloaded by [University of Technology Sydney] at 18:10 07 April 2015

Methods Study sites We studied four tributaries of the Hunter River in New South Wales, Australia: the Allyn, the Chichester, the Paterson and the Williams rivers (Figure 1). These rivers rise on the Barrington Plateau at approximately 1500 m altitude and ﬂow in a generally southeasterly direction. The region has primarily Carboniferous sedimentary and volcanic geology and a warm temperate climate (mean annual temperature of 18C), with median annual rainfall of 1061–1278 mm concentrated in the austral summer (40% falling in January–March) but with high inter-annual variability. The upper reaches of each river lie in relatively undisturbed catchments in either a national park or state forest, whereas grazing and dairy Journal of Freshwater Ecology 71

Figure 1. Location of collection sites; numbers indicate reaches referred to in the text.

production are the major land uses along the lower reaches. The riparian vegetation in the lower reaches is dominated by native species including river bottlebrush (Callistemon sie- beri), river oak (Casuarina cunninghamiana) and spiky mat rush (Lomandra spp.) as well as exotic species including willows (Salix spp.) and giant reed (Arundo donax). Two dams were included in the study: Chichester Dam (built in 1921, 43 m height, Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 21 106 m3 capacity) on the Chichester River which supplies water for domestic con- sumption and Lostock Dam (1971, 38 m height, 20 106 m3 capacity) on the Paterson River which supplies water for agricultural purposes (mainly irrigation). Both dams have a small operational capacity relative to annual inflow volume, so storage levels rarely fall below 80% and spills are frequent. They can develop thermal and oxygen stratification (particularly in summer because of their depth and consistently high storage levels) and discharge cold water to downstream river reaches through hypolimnetic outlets. Four sites, each a 100 m reach, were chosen on each of the Chichester, the Paterson and the Williams rivers and three on the Allyn River (Table 1, Figure 1). One site was selected in the sub-alpine headwaters of each river at 1300 to 1480 m altitude, except for the Allyn River where difficult terrain prevented access. A second site was established within the national park or state forest on the slope reaches of each river at 240–350 m. Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 72 .Growns I. tal. et Table 1. Characteristics of sites identified in Figure 1.

Median annual River Reach number Longitude Latitude Altitude (m) Distance from source (km) Catchment area (km2) discharge (x106 m3)

Allyn 2 151 300 E32 100 S 310 17 56 9.1 Allyn 3 151 330 E32 220 S 90 52 241 27.2 Allyn 4 151 340 E32 270 S 37 66 292 39.6 Chichester 1 151 300 E32 40 S 1300 5 8 0.9 Chichester 2 151 350 E32 100 S 240 15 60 9.1 Chichester 3 (Dam) 151 420 E32 140 S 143 35 204 30.0 Chichester 4 151 430 E32 180 S 85 44 232 32.5 Paterson 1 151 250 E32 40 S 1480 2 2 0.7 Paterson 2 151 250 E32 110 S 350 19 52 7.4 Paterson 3 (Dam) 151 270 E32 200 S 141 51 260 25.2 Paterson 4 151 330 E32 310 S 25 107 453 28.7 Williams 1 151 290 E32 40 S 1360 3 3 1.4 Williams 2 151 320 E32 100 S 340 17 46 7.0 Williams 3 151 430 E32 180 S 90 51 203 24.4 Williams 4 151 460 E32 280 S 36 82 720 82.3 Modelled data from Stein et al. (2007). Journal of Freshwater Ecology 73

A third site was located immediately downstream of the Chichester and Lostock dams on the Chichester and the Paterson rivers, respectively, and at similar altitudes on the Allyn and the Williams rivers. A fourth site was established on each river at the most downstream accessible location with both pools and rifﬂes. The fourth site on the Williams River was considered to be unaffected by the Chichester Dam because 80% of its catchment area was unregulated.

Field sampling We collected invertebrates and potential food sources at each site between November 2008 and March 2009. All sites were only sampled once between those months and all material was collected on the same day at each site. New leaf growth of the dominant riparian vegetation was collected by hand. Biofilm samples were collected from the pools and riffles by scrubbing rocks in a 10-L bucket filled with filtered river water and the resulting suspension being filtered through a 250-mm mesh sieve with biofilm retained by a 25-mm mesh net. Benthic organic matter was collected by washing sediment with river water and retaining organic material on graded sieves as coarse particulate organic matter (CPOM, 2–5 mm) and fine particulate organic matter (FPOM, 0.25–2 mm). Samples of filamentous algae, if present, were collected by hand from rocks. Macroinvertebrates were collected from pools and riffles at each site with a 250-mm mesh dip net and by handpicking from rocks taken from the river bed. Representatives of the most abundant taxa at each site were immediately preserved in ethanol for GCA. Sam- ples for SIA were stored in plastic zip-lock bags and immediately placed on ice for at least 8 hours. This procedure allowed the invertebrates to void their guts, removing unassimi- lated material. Samples were frozen at 20C upon return to the laboratory prior until further processing.

Gut contents analysis GCA followed the methods of Chessman (1986). Preserved invertebrates were identiﬁed to the lowest possible taxon using keys in Hawking (2000) and dissected individually under a stereomicroscope. A maximum of 10 individuals per taxa per site were examined, selected where possible to span a range of body sizes. Each specimen was washed with distilled water and, if necessary, adhering debris was removed with a small brush and forceps. Invertebrates were then dried with tissue and the thorax and abdomen were slit with ﬁne forceps and needles. The anterior half of the digestive tract was removed, and its con-

Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 tents were expelled into a droplet of distilled water on a microscope slide and distributed as uniformly as practical. A cover slip was placed over the droplet which was then scanned at magnifications of 100x–400x under a compound microscope. Food items were classified into seven categories: unidentifiable fine organics, fungi, planktonic algae, non-filamentous benthic algae (mostly diatoms), filamentous algae, plant material (wood and leaf fragments) and animals (invertebrate fragments). Inorganic material was not included in the analysis. The food categories observed in each digestive tract were ranked in the order of increasing abundance, assessed subjectively according to the area of the slide covered. Points were then awarded to each category by expressing its rank as a proportion of the sum of the ranks of all categories in the same specimen. The gut contents of 503 invertebrates from 30 taxa were examined but several of these were collected from only one or a few sites. Sixteen taxa were sufficiently abundant and distributed amongst enough sites to be included in the analysis (Table 2). Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 74 .Growns I.

Table 2. Average percentage ( 1 S.E.) of the total points for seven food categories in 16 invertebrate taxa. al. et

Unidentiﬁable Total number of No. of ﬁne organic Benthic Planktonic Filamentous Vascular Family Taxon individuals sites material algae algae algae Fungi plant material Animal matter

Aytidae Paratya australiensis 82 11 70.6 2.2 21.9 1.8 – 2.8 0.8 0.2 0.2 4.5 1.1 0.1 0.1 Baetidae Edmundsiops spp. 35 5 47.1 3.1 46.7 3.3 – 6.2 1.9 – – – Hydropsychidae Asmicridea AV1 23 6 23.5 2.7 24.2 3.6 – 19.9 3.2 0.3 0.3 9.3 2.7 22.9 4.3 Cheumatopsyche AV1 10 1 24.1 4.1 17.6 1.7 – 10.1 2.5 1.0 1.0 26.9 2.6 20.4 3.6 Cheumatopsyche AV2 10 1 43.7 4.4 29.7 2.4 – 17.7 4.7 – 7.0 3.6 2.0 2.0 Cheumatopsyche AV6 35 4 31.8 2.6 21.5 1.9 0.9 0.0 16.3 1.8 – 16.0 2.7 13.5 2.5 Diplectrona AV3 18 3 18.3 1.3 1.1 0.6 – 3.7 1.5 1.1 1.1 41.9 3.0 33.9 2.4 Leptoceridae Notalina spira 12 5 22.5 2.2 34.4 4.7 – 27.8 6.5 0.8 0.8 11.1 5.4 3.3 3.3 Triplectides altenogus 4 2 32.5 0.8 9.2 5.3 – – 2.5 2.5 55.8 6.6 – Triplectides australicus 56 8 19.0 1.6 13.8 2.0 – 16.8 3.5 4.2 1.3 46.2 3.4 – Triplectides AV10 4 2 33.3 0 4.2 4.2 – 4.2 4.2 – 58.3 4.8 – Triplectides ciuskus 14 3 27.4 2.8 4.8 3.2 – – 2.4 2.4 65.5 3.3 – Triplectides parvus 18 4 22.6 3.3 14.6 4.0 – 1.9 1.3 0.4 0.4 60.6 6.0 – Triplectides similis 13 3 25.1 7.2 – – 6.2 4.0 6.7 3.6 40.8 8.1 21.3 9.0 Leptophlebiidae Austrophlebioides spp. 102 11 51.0 2.0 39.9 2.0 – 5.1 1.0 0.5 0.4 3.0 0.8 0.5 0.3 Nousia spp. 22 8 57.6 4.7 15.2 5.0 – 0.5 0.5 6.8 2.4 20.0 3.5 – Journal of Freshwater Ecology 75

Stable isotope analysis Leptocerid cases and shrimp exoskeletons were removed prior to isotope analysis. All samples were dried at 60C for 24–48 hours, ground to a powder with a glass rod and por- celain dish, and pelletised in tin capsules. Pelletised samples were analysed for stable isotopes with a continuous ﬂow-isotope ratio mass spectrometer (Eurovector EA3000, Milan, Italy) at Grifﬁth University, Brisbane, Australia. The ratios (R) of the heavy isotope 13C to the light isotopes 12C and 15Nto14N were expressed in parts per thousand, relative to standards (Pee Dee belemnite limestone and atmospheric nitrogen, respectively) in delta notation according to the following equation:

13 15 d Cord N ¼½ðRsample=RstandardÞ1 1000

Spatial patterns in consumer stable isotopes were analysed for the following invertebrate taxa or groups: Paratya australiensis, Hydropsychidae, Leptoceridae, Ephemerop- tera nymphs and predators (Gyrinidae, Odonata and Megaloptera).

Statistical analysis Differences in the gut contents between the selected taxa were tested with analysis of sim- ilarities (ANOSIM) and the Bray–Curtis dissimilarity measure (Clarke 1993) in the PRIMER program (Clarke & Gorley 2006). This was done in order to group those with similar diets for longitudinal analysis, which was necessary because of the patchy distri- butions of individual species. Dietary items that contributed most to significant differences were identified with similarity percentages (SIMPER) in PRIMER. We used linear regression to assess the longitudinal changes in the importance of the gut contents of common species or groups of species with similar diets. The gut contents data (points per food category) were averaged for all individuals of a species or group at each site. Principal components analysis was used to summarise the variability in the gut contents across all sites, excluding those immediately downstream of the dams. The first principal component of the gut contents for each species or group was regressed against the distance of each site from the source of the river. If a significant longitudinal relationship was not observed, ANOSIM was used to test whether the gut contents of invertebrates collected immediately downstream of the dams were significantly different from those at the remaining sites. Longitudinal changes in d13C and d15N of the invertebrate groups and potential food d13 d15

Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 were also assessed with linear regression. The Cor N values of the invertebrate groups and food at each site, excluding the sites downstream of the dams, were regressed on distance from the source of the river. If a significant relationship was observed between the distance from the source and d13Cord15N of an invertebrate group or food; the data were plotted with 95% confidence limits. We inferred that the dams affected the isotopic composition if the d13Cord15N values lay outside the confidence limits of the regression line. Where no significant longitudinal variation could be established we compared the d13Cord15N values between the sites immediately downstream of the dams and the remaining sites with one-way analysis of variance. The contribution of potential food sources to primary consumers was assessed for each river reach and sites below the dams using the MixSIR mixing model (Semmens & Moore 2008). This algorithm carries out Bayesian analysis using sampling importance resampling and is able to explicitly account for uncertainty in the isotope source and 76 I. Growns et al.

fractionation values (Moore & Semmens 2008; Semmens et al. 2009; Jackson et al. 2009). Fractionation constants were estimated from the literature and were set at 0.4 1.20 % (m S.D.) and 2.3 1.61 % for d13C and d15N, respectively (McCutchan et al. 2003).

Results Gut contents analysis The shrimp Paratya australiensis was collected at all sites except those in reaches 1 and 2 on the Chichester River and reach 1 on the Paterson and the Williams rivers (Table 2). Its gut contents consisted mainly of unidentifiable organic matter and unicellular benthic algae and contained small amounts of the remaining food types, excluding planktonic algae (Table 2). The first principal component explained 60% of the variation of the shrimp diet data and there was no significant longitudinal trend in the gut contents of P. australiensis (linear regression, F < 1.0, p > 0.10) and no difference between the sites immediately downstream of the dams and the remaining sites (ANOSIM, rho ¼0.07, p ¼ 0.82). The baetid mayfly nymph Edmundsiops spp. was collected from all four rivers but only at five sites (Table 2). Its gut contents consisted mainly of unidentifiable organic matter and unicellular benthic algae, but a small amount of filamentous algae was also present (Table 2). The gut contents of Edmundsiops spp. were similar to those of the leptophlebid mayfly Austrophlebioides spp. at sites not immediately below the dams (rho ¼0.04, p ¼ 0.71), but significantly different from those of the leptophlebiid mayfly Nousia spp. (rho ¼ 0.37, p ¼ 0.003). The gut contents of the two leptophlebiid species were also significantly different (rho ¼ 0.41, p < 0.001). SIMPER analysis identified that unicellular benthic algae (38%), unidentified fine organic material (26%) and vascular plant material (22%) contributed to the dissimilarity between Austrophlebioides spp. and Nousia spp. The gut contents of Austrophlebioides spp. contained more unicellular benthic algae and unidentified fine organic material and less vascular plant material than Nousia spp. Therefore, Edmundsiops spp. and Austrophlebioides spp. but not Nousia spp. were combined to allow sufficiently wide spatial representation for analysis of patterns in the mayfly diets. The first principal component explained 85% of the variation of the diet data and there was no significant longitudinal change in the gut contents of Edmundsiops spp. plus Austrophlebioides spp. (F ¼ 0.12, p > 0.10) and no difference between the sites immediately downstream ¼ ¼

Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 of the dams and the remaining sites (rho 0.10, p 0.97). Hydropsychid caddisflies occurred at approximately half of the sites (Table 2), with gut contents differing significantly among species (rho ¼ 0.30, p < 0.001). Pair-wise comparisons indicated that the gut contents of Diplectrona spp. were significantly different from those of the remaining species. SIMPER analysis identified that vascular plant material (28%), animal fragments (21%) and unicellular benthic algae (21%) contributed to the dissimilarity between the gut contents of Diplectrona spp. and the other species. The gut contents of Diplectrona spp. contained more animal and vascular plant material and less unicellular benthic algae. With combined data for all species except Diplectrona spp., the first principal component explained 87% of the variation of the diet data and there was no significant longitudinal trend in the gut contents (F ¼ 0.34, p > 0.10) and no difference between sites immediately downstream of dams and the remaining sites (rho ¼ 0.04, p ¼ 0.12). Journal of Freshwater Ecology 77

Leptocerid caddisflies were collected from all reaches except the headwater sites (Table 2). There was no significant difference in the gut contents of the species (rho ¼ 0.08, p ¼ 0.06). The first principal component explained 66% of the variation of the diet data but there was no significant longitudinal pattern in the gut contents of all the species combined (F ¼ 1.7, p > 0.10) or difference between sites immediately downstream of dams and the remaining sites (rho ¼0.06, p ¼ 0.83).

Stable isotopes Significant longitudinal changes were observed in d13CofP. australiensis and FPOM (Figure 2) but not in the other invertebrate groups and potential food sources. The d13C values of P. australiensis decreased with increasing distance from the source (F ¼ 18.4, p < 0.01) whereas those of FPOM increased with increasing distance from the source (F ¼ 11.1, p < 0.01). The d13C values of P. australiensis in reaches 3 and 4 downstream of the dams fell outside the confidence limits of the regression but in different directions, indicating no consistent dam effects. There were also no consistent dam effects on the d13C values for FPOM at the downstream sites. The d13C value for the filamentous algae was significantly higher downstream of the Lostock Dam than at the remaining sites (F ¼ 11.8, p < 0.05, Figure 2) but d13C values of other invertebrates and food sources did not differ significantly between the dam and no-dam sites (F < 2.0, p > 0.05). d15N values increased with increasing distance from the water source for all invertebrate groups, CPOM, riparian plants, pool biofilm and filamentous algae (F > 17.0, Downloaded by [University of Technology Sydney] at 18:10 07 April 2015

Figure 2. Scatterplots of mean d13C values against the distance from the source. Regression lines with lower and upper 95th percentile conﬁdence limits are shown for signiﬁcant relationships. C3, P3, C4 and P4 refer to reaches 3 and 4 on the Chichester and the Paterson rivers, respectively. 78 I. Growns et al.

Figure 3. Scatterplots of mean d15N values against the distance from the source. Regression lines with lower and upper 95th percentile conﬁdence limits are shown for signiﬁcant relationships. C3, P3, C4 and P4 refer to reaches 3 and 4 on the Chichester and the Paterson rivers, respectively.

p < 0.01, Figure 3). The d15N values at reaches 3 and 4 on the Chichester River and reach 3 on the Paterson River, all within 9 km of the dams, lay above the regression confidence limits, implying significantly higher d15N, in all cases except CPOM and pool biofilm. In contrast, the d15N values of those biota and potential food sources at reach 4 on the Paterson River, 56 km from the dam, fell within the confidence limits, implying the dam effect had no effect this far downstream. The d15N values of FPOM were significantly higher at the dam sites than elsewhere (F ¼ 6.1, p < 0.05, Figure 2). In contrast, there was no significant difference in the d15N values of riffle biofilms between dam and no-dam sites (F ¼ 0.7, p > 0.10). Downloaded by [University of Technology Sydney] at 18:10 07 April 2015

Mixing model The contribution of the autochthonous sources (filamentous algae and biofilms) to the diet of P. australiensis decreased with increasing river size and was similar at dam sites compared with the same river section (section 3) on the unregulated rivers (Figure 4). Simi- larly, allochthonous contributions to the diet of mayflies increased with increasing river size, principally due to a decrease in the filamentous algae contributions. However, the contribution of the food sources at sites downstream of the dams was different from the same section of river on the unregulated rivers. There were no consistent longitudinal patterns in the contributions of the various food sources for either Leptoceridae or Hydropsychidae. Journal of Freshwater Ecology 79

Figure 4. Median percent contribution of ﬁve potential food sources to the diets of four primary consumers in three separate river sections and sites immediately downstream of dams.

Discussion Most invertebrate groups and potential food sources in our study rivers had significant longitudinal trends in d15N and there was 15N enrichment downstream of the dams that exceeded expectations from the dams’ longitudinal position relative to other sites. The observation of enrichment is supported by Maxwell (2010), who observed enrichment of 15N in biofilm downstream of the Chichester and Lostock dams. The processes that gov- ern the ratios of nitrogen isotopes are complex (Hoegberg 1997; Nestler et al. 2011) but increased anthropogenic nitrogen inputs to aquatic systems often result in enrichment of 15N in biota (e.g. Costanzo et al. 2001). Consequently, the longitudinal 15N enrichment in our rivers may have been the result of increased agricultural land-use in their lower reaches. However, this mechanism is unlikely as Miyajima et al. (2009) found no effect of land use or geology on longitudinal 15N enrichment of suspended particulate nitrogen Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 or nitrates. Udy and Bunn (2001) found 15N enrichment of aquatic macrophytes but not riparian plants at sites with greater catchment clearing. Our results for riparian plants are also in contrast with previous findings for terrestrial plants, which generally show d15N depletion with decreasing altitude (e.g. Sah & Brumme 2003; Liu & Wang 2010). Longitudinal changes in d15N may be caused by several mechanisms, including isotope fractionation during in-stream nitrogen removal by denitrification and assimilation, in-stream nitrification generating isotopically different nitrogen relative to upstream sites, and external loading of isotopically different nitrogen from upstream sources (Miyajima et al. 2009). Information on these processes is not available for the four rivers in the present study and, consequently, the mechanisms underlying their longitudinal 15N enrichment require further investigation. Many studies suggest that dams are net export- ers of nitrogen and other authors have also demonstrated enriched 15N in the biota 80 I. Growns et al.

downstream of dams (Moore et al. 1992;Xuetal.2005; Beutel 2006;Dudaetal.2010). It is likely that the enrichment of 15N that we observed downstream of the dams is due to reservoir processes releasing isotopically enriched nitrogen compounds to downstream reaches. In contrast to our results for d15N, we could not detect systematic longitudinal variation in the d13C values of the majority of invertebrates and potential food sources or effects of dams on these variables. The lack of spatial variation in d13C is supported by our observed lack of spatial variation in the gut contents of the primary invertebrate consumers. However, we observed a decreasing contribution of autochthonous food sources to the diet of P. australiensis, which was supported by a trend of 13C depletion with increasing distance from source. The lack of longitudinal variation in d13C values in the majority of invertebrates, CPOM, FPOM and riparian plants is consistent with the findings of Finlay (2001) and Hadwen et al. (2010a). However, the lack of longitudinal trends in d13C of epilithon in the present study is in agreement with the findings of Hadwen et al. (2010a) but in contrast to those of Finlay (2001). The absence of longitudinal trends in biofilm d13C in our rivers may have been due to high variability in the d13C values among species of algae and other biota that make up the biofilm (Hadwen et al. 2010b). An alter- native explanation is that the factors that influence fractionation of d13C by algae, such as carbon supply and photosynthetic rates, may be too spatially or temporally (sampling took place over several months) variable across our rivers to generate consistent longitudinal patterns. The minimal influence of dams on the d13C values of potential food sources and biota in this study is in contrast to other studies that have demonstrated enrichment or depletion of 13C in biota downstream of dams (Angradi 1993, 1994; Shannon et al. 2001; Doi et al. 2008). The influence of the Lostock and Chichester dams on downstream d13C values may be minor because they are small dams that spill frequently with hypolimnetic water that is infrequently released. The effects of dams on river hydrology and, therefore, ecology, vary according to the structural features of the impoundment, the purpose of the dam and how it is operated (Armitage 1984; Finlayson et al. 1994). The lack of a consistent effect of dams on 13C in the literature may reflect differing management of the dams in previous studies. However, the inconsistent effect suggests that there may be numerous mechanisms operating to influence carbon isotope dynamics and therefore d13C values of biota. In conclusion, longitudinal increases in 15N enrichment were observed from the headwaters to the lowland reaches of our study rivers in the majority of invertebrate groups and potential food sources, but little systematic spatial variation in gut contents d13

Downloaded by [University of Technology Sydney] at 18:10 07 April 2015 or C signatures was noted. Additionally, most invertebrate groups and food sources were 15N enriched immediately downstream of the dams, but the impoundment of water did not appear to influence either the gut contents of primary consumers or d13C signatures of invertebrates and potential food. The observed 15N enrichment downstream of the dams was most likely due to the effects of reservoir processes on water chemistry and further research is required to elucidate the mechanisms responsible for the observed longitudinal trends in d15N. Our observations regarding the influence of dams on isotope composition and energy flow are contrary to the few previous similar studies, perhaps because the dams in the present study have small operational capacities relative to annual inflows, resulting in reduced downstream impacts of regulated flow regimes relative to the size of the reservoirs in the other studies. We recommend that future studies incorporate multiple dams of different sizes or operational rules to test this hypothesis. Journal of Freshwater Ecology 81

Acknowledgements We thank Mark Dahm, David Hoey, David Ryan and Mark Simons for ﬁeld assistance, and Rene Diocares at Grifﬁth University for analysing the stable isotope samples. Dr Darren Ryder and anonymous reviewers are thanked for comments on the manuscript.

Funding This study was jointly funded by the Land and Water Australia Innovation Grant Scheme and the New South Wales Ofﬁces of Water and Environment and Heritage.

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